1. Field of the Invention
The present invention relates to a recording paper surface detection apparatus that detects surface conditions of a recording sheet based on a surface image formed by an image capture device, and also to an image forming apparatus that controls image forming conditions based on the result of recording paper surface detection.
2. Description of the Related Art
With image formation apparatuses such as copying machines and laser beam printers, a developed image made visible by a developing device is transferred onto a recording sheet under predetermined transfer conditions, and heat and pressure are applied to the recording sheet under predetermined fixing conditions to fix the transferred image thereon.
In thus-configured conventional image forming apparatuses, for example, the size and type of recording sheet are set by the user on an operation panel provided thereon (hereinafter, the type of recording sheet is referred to as paper type). Transfer conditions (including transfer bias and conveyance speed of the recording sheet at the time of image transfer) and fixing conditions (including fixing temperature and conveyance speed of the recording sheet at the time of fixing) are controlled based on the setting on the operation panel.
Japanese Patent Application Laid-Open No. 2004-38879 discusses a technique including: capturing surface conditions of a recording sheet by using an area sensor; detecting the surface smoothness from the captured surface image; determining the paper type of the recording sheet; and controlling transfer conditions and/or fixing conditions based on the result of paper type determination. This technique for capturing a surface image of the recording sheet by using an area sensor excels in paper type determination accuracy since the shadow produced by the surface unevenness is directly captured. In particular, the method provides a high paper type determination accuracy when the presence or absence, size, and depth of the surface unevenness can be definitely distinguished in visual way, for example, when distinguishing between coated paper and non-coated paper (plain paper).
However, when determining the paper type of a general recording sheet for office use, for example, shadow conditions produced by the surface unevenness largely depend on the direction of fiber arrangement, i.e., paper making (hereinafter referred to as fiber arrangement direction). More specifically, when the paper surface is illuminated with light from a direction of irradiation that perpendicularly intersects with the fiber arrangement direction, a captured image provides high contrast emphasizing the surface unevenness. However, when the paper surface is illuminated with light from the same direction as the fiber arrangement direction, a captured image provides low contrast because of indistinct shadow produced by the surface unevenness. Therefore, even for the same type of paper, the contrast of the captured image largely differs leading to different results of paper type determination between longitudinal and lateral sheet passing.
With the technique in Japanese Patent Application Laid-Open No. 2004-38879, the recording sheet is illuminated with light from an oblique direction, specifically, at 15 to 70 degrees with respect to the recording sheet conveyance direction on the premise that the fiber arrangement direction of almost all paper types fits into an angular range from 0 (coincidence) to ±15 degrees with respect to the recording sheet conveyance direction or a direction perpendicularly intersecting with it. An image of this light illuminated area is captured by the area sensor to improve the paper type determination accuracy. However, the fiber arrangement direction of the recording sheet depends on the compounding rate of raw materials in the manufacture process. In recent years, however, recording paper has been produced through diverse manufacture processes on various manufacture sites, which results in diverse fiber arrangement directions. Therefore, the fiber arrangement direction does not necessarily fit into the angular range from 0 (coincidence) to ±15 degrees with respect to the longitudinal or lateral direction of the recording sheet. Therefore, with the technique in Japanese Patent Application Laid-Open No. 2004-38879 in which a surface image of one area illuminated with light from one direction is captured, an identical recording sheet may give different results of paper type determination depending on a relation between the fiber arrangement direction and the light illumination direction.